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Indicator   A substance which takes on a different colour dependin on the pH of a solution.

Common indicators include litmus (red in acid, blue in alkali) and phenolphthalein (colourless in acid, pink in alkali). Many natural substances can act as an indicator (eg red cabbage, blackcurrant juice)

 Full-range indicator and universal indicator are mixtures of different indicators. The mixtures ae carefully designed to give a gradual colur change from acid to alkali. They are available as solutions or papers which can be matched to a colour chart to tell you the pH.

Ion   An atom or group of atoms that has gained or lost one or more elecrons.

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